Apparatus and method for inspecting a sample using a plurality of charged particle beams

Abstract

Disclosed is an apparatus and method for inspecting a sample. The apparatus includes: a sample holder, a multi beam charged particle generator for generating an array of primary charged particle beams, an electro-magnetic lens system for directing the array of primary charged particle beams into an array of separate focused primary charged particle beams on the sample, a multi-pixel photon detector arranged for detecting photons created by the focused primary charged particle beams when the primary charged particle beams impinge on the sample or after transmission of the primary charged particle beams through the sample, and an optical assembly for conveying photons created by at least two adjacent focused primary charged particle beams of the array of separate focused primary charged particle beams to distinct and/or separate pixels or groups of pixels of the multi-pixel photon detector.

Claims

1. An apparatus for inspecting a sample, wherein the apparatus comprises: a sample holder for holding the sample, a multi beam charged particle generator for generating an array of primary charged particle beams, an electro-magnetic lens system for directing said array of primary charged particle beams into an array of separate focused primary charged particle beams at said sample holder, a multi-pixel photon detector arranged for detecting photons created by said focused primary charged particle beams when said primary charged particle beams impinge on the sample or after transmission of said primary charged particle beams through the sample, and an optical assembly for conveying photons created by at least two adjacent focused primary charged particle beams of said array of separate focused primary charged particle beams to distinct and/or separate pixels or to distinct and/or separate groups of pixels of the multi-pixel photon detector, wherein said apparatus comprises a layer of cathodoluminescent material, wherein the sample holder is arranged to position the sample between the electro-magnetic lens system and the layer of cathodoluminescent material, such that the charged particles impinge on the layer of cathodoluminescent material after transmission through said sample, wherein said apparatus is configured to distinguish between 0.sup.th order transmitted charged particles and scattered charged particles.

2. The apparatus according to claim 1, wherein the sample holder is arranged to position the sample spaced apart from said layer of cathodoluminescent material.

3. The apparatus according to claim 1, wherein the multi-pixel photon detector is configured to distinguish between light generated in the layer of cathodoluminescent material by 0.sup.th order transmitted charged particles and light generated in the layer of cathodoluminescent material by scattered charged particles.

4. The apparatus according to claim 3, wherein multi-pixel photon detector comprises light sensitive areas, wherein the light sensitive areas comprising: a centre light sensitive area for detecting the light generated by the 0.sup.th order transmitted charged particles, and a ring of light sensitive areas arranged around the centre light sensitive area for detecting the light generated by the scattered charge particles.

5. The apparatus according to claim 3, wherein the multi-pixel photon detector comprises pixels arranged in rows and columns, wherein the detector is configured to detect central light spots which originate from the 0.sup.th order transmitted charged particles, and to separate and detect surrounding light spots that originate from scattered charged particles and which surrounding light spots surround at least one of the central light spots.

6. The apparatus according to claim 1, wherein the optical assembly comprises a filtering device which is arranged for blocking light generated by the 0.sup.th order transmitted charged particles or light generated by the scattered charged particles.

7. The apparatus according to claim 1, wherein the layer of cathodoluminescent material is supported by a light transmitting support plate.

8. The apparatus according to claim 1, wherein said layer of cathodoluminescent material is covered with a charge conducting layer.

9. The apparatus according to claim 8, wherein the charge conducting layer is arranged at a side of said layer of cathodoluminescent material facing the electro-magnetic lens system.

10. The apparatus according to claim 1, wherein said optical arrangement comprises a lens system which is arranged for imaging said created photons onto the multi-pixel photon detector with an optical magnification between 5 and 500.

11. The apparatus according to claim 1, wherein the electro-magnetic lens system is arranged to project an array of separate spots on the sample surface where the focused primary charged particle beams impinge on the sample on the sample holder, wherein the pitch between the spots on the sample surface is between 0.3 and 30 micrometers.

12. The apparatus according to claim 1, wherein the multi-pixel photon detector is a CCD camera, a CMOS camera, an array of avalanche photo diodes or an array of photo multipliers.

13. The apparatus according to claim 12, wherein the CCD camera, CMOS camera, array of avalanche photo diodes or photo multipliers comprises an array of detector pixels is positioned such that the array of detector pixels coincides with an array of images of the individual light spots created by the individual beams of said primary charged particle beams.

14. The apparatus according to claim 1, further comprising a system for scanning the focused primary charge particle beams over said sample holder, and a control-and-signal-processing system for creating one image per primary charged particle beam.

15. The apparatus according to claim 1, further comprising a first actuating system for moving the sample holder at a constant speed in a first direction, and a second actuating system for scanning the focused primary charge particle beams over said sample holder in a second direction at least substantially perpendicular to the first direction, and a control-and-signal-processing system for creating one image per primary charged particle beam.

16. The apparatus according to claim 12, wherein said apparatus further comprises a signal processing unit for combining said individual images per primary charged particle beam into one combined image of at least a part of the sample.

17. A method for inspecting a sample, comprising the steps of: arranging a sample on a sample holder, generating an array of primary charged particle beams using a multi beam charged particle generator, directing said array of primary charged particle beams into an array of separate focused primary charged particle beams on said sample in or on the sample holder using an electro-magnetic lens system, directing at least part of the charged particles which pass through the sample to a layer of cathodoluminescent material, wherein the sample holder positions the sample between the electro-magnetic lens system and the layer of cathodoluminescent material, converting at least part of the charged particles which have passed through the sample and impinge of the layer of cathodoluminescent material into photons, detecting photons created by said focused primary charged particle beams when said primary charged particle beams impinge on the sample or after transmission of said primary charged particle beams through the sample using a multi-pixel photon detector, conveying the photons created by at least two adjacent focused primary charged particle beams of said array of separate focused primary charged particle beams to distinct and/or separate pixels or to distinct and/or separate groups of pixels of the multi-pixel photon detector using an optical assembly, and wherein the multi-pixel photon detector distinguishable detects 0.sup.th order transmitted charged particles and/or scattered charged particles.

18. The method according to claim 17, wherein the apparatus obtains a bright field image and/or a dark field image.

19. The method according to claim 17, wherein said sample holder positions the sample spaced apart from said layer of cathodoluminescent material.

20. The method according to claim 17, wherein the optical assembly comprises a filtering device which blocks light generated by the 0.sup.th order transmitted charged particles or light generated by the scattered charged particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be elucidated on the basis of an exemplary embodiment shown in the attached drawings, in which:

(2) FIG. 1 shows an example of a Multi-Beam Scanning Electron Microscope (MBSEM),

(3) FIG. 2 shows a first embodiment of a detection arrangement according to the invention,

(4) FIG. 3 shows the arrangement of the sample on top of a cathodoluminescent layer,

(5) FIG. 4 shows a second embodiment of a detection arrangement according to the invention,

(6) FIG. 5 shows the arrangement of the sample at a distance from a cathodoluminescent layer,

(7) FIG. 6 shows an example of a segmentation of the detection area of a detector,

(8) FIGS. 7A and 7B show an example of a thin cathodoluminescent layer on top of a light transmitting support plate,

(9) FIG. 8 shows an example of a cathodoluminescent layer provided with a charge conducting layer, and

(10) FIG. 9 shows a further example of an embodiment of a detection arrangement with a filtering device.

DETAILED DESCRIPTION OF THE INVENTION

(11) FIG. 1 shows an example of a Multi-Beam Scanning Electron Microscope (MBSEM) of the invention. The MBSEM 1 comprises a multi beam charged particle generator 2 for generating an array of primary charged particle beams, in this case an array of primary electron beams 3. The multi beam electron generator 2 comprises at least one electron source 4 for generating a diverging electron beam 5. The diverging electron beam 5 is split into an array of focused primary electron beams 3 by an aperture lens array 6. The primary electron beams 3 are subsequently directed towards a sample 15 in a sample holder 150, as schematically indicated by the arrow P.

(12) The multiple images of the source 4 are positioned on the object principle plane of an accelerator lens 7. The accelerator lens 7 directs the primary electron beams 3 towards the optical axis 8 and creates a first common cross-over 9 of all the primary electron beams 3.

(13) The first common cross-over 9 is imaged by the magnetic condenser lens 10 onto a variable aperture 16 that acts as a current limiting aperture. At the variable aperture 16, a second common cross-over of all the primary electron beams 3 is created.

(14) The MBSEM comprises a lens system 13, 14 for directing the primary charged particle beams from the common cross-over at the variable aperture 16 towards the sample surface 15 and for focusing all primary charged particle beams 3 into an array of individual spots on the sample surface 15. The lens system comprises an intermediate magnetic lens 13 for imaging the variable aperture 16 onto a coma free plane of the objective lens 14, which objective lens 14 creates an array of focused primary electron beams on the sample surface 15.

(15) In addition the MBSEM is provided with scan coils 18 for scanning the array of focused primary electron beams over the sample surface 15.

(16) The MBSEM thus comprises a multi beam charged particle generator 2 for generating an array of primary charged particle beams 3, and an electro-magnetic lens system 13, 14 for directing said array of primary charged particle beams 3 into an array of separate focused primary charged particle beams at said sample 15 in the sample holder 150.

(17) According to the invention, and as schematically shown in a first example of FIG. 2, the apparatus further comprises a multi-pixel photon detector 20 arranged for detecting photons 30, 31, 32 created by said focused primary charged particle beams 3, 3, 3 when said primary charged particle beams 3, 3, 3 impinge on the sample 15. In addition the apparatus comprises an optical assembly 40 for conveying photons 30, 31 created by at least two adjacent focused primary charged particle beams 3, 3 of said array of separate focused primary charged particle beams 3, 3, 3 to distinct and/or separate pixels or to distinct and/or separate groups of pixels of the multi-pixel photon detector 20.

(18) When the sample 15 comprises one or more cathodoluminescent constituents, photons 30, 31, 32 are created by said focused primary charged particle beams 3, 3, 3 when said primary charged particle beams 3, 3, 3 impinge on the sample 15, in particular on the cathodoluminescent constituents thereof.

(19) However, when the sample 15 does not generate sufficient photons 30, 31, 32 to be readily detected by the multi-pixel photon detector 20, a layer of cathodoluminescent material 19 is arranged at a side of the sample 15 facing away from the electro-magnetic lens system 13, 14, such that the charged particles from the primary charged particle beams 3, 3, 3 impinge on the layer of cathodoluminescent material 19 after transmission through said sample 15. In the example as shown in FIG. 3, the sample holder 150 is arranged to position the sample 15 in direct contact with and supported by said layer of cathodoluminescent material 19. When the charged particles from the primary charged particle beams 3, 3, 3 hit the layer of cathodoluminescent material 19, photons are created from an interaction volume 190. The size of this interaction volume 190 and the intensity of the generated light, depends, inter alia, on the energy of the charged particles which impinge of the layer of cathodoluminescent material 19.

(20) In a second example of the apparatus of the present invention, as schematically shown in FIG. 4, the sample holder 150 is arranged to position the sample 15 at a distance d from said layer of cathodoluminescent material 19. Again, the apparatus comprises a multi-pixel photon detector 20 arranged for detecting photons 30, 31, 32 created by charged particle beams 23, 23, 23 when they impinge on the layer of cathodoluminescent material 19 after transmission of the primary focused charged particle beams 3, 3, 3 through sample 15. The apparatus also comprises an optical assembly 40 for conveying photons 30, 31 created by at least two adjacent charged particle beams 23, 23 of said array of transmitted charged particle beams 23, 23, 23 to distinct and/or separate pixels or to distinct and/or separate groups of pixels of the multi-pixel photon detector 20.

(21) The arrangement of the sample 15 and the layer of cathodoluminescent material 19 is shown in more detail in FIG. 5. This figure shows furthermore that the charged particles of the primary focused charged particle beams 3, 3, 3 can travel straight through the sample 15. These charged particles are also denoted as 0.sup.th order transmitted charged particles. When the 0.sup.th order transmitted charged particles hit the layer of cathodoluminescent material 19, photons are created from an interaction volume 190. When using the photons created by these 0.sup.th order transmitted charged particles to make an image of the sample, a bright field image is obtained.

(22) In addition, the charged particles of the primary focused charged particle beams 3, 3, 3 can also be scattered by the sample 15. These scattered charged particles exit the sample 15 at an angle with respect to the straight traveling 0.sup.th order transmitted charged particles. Because of the distance d between the sample 15 and the layer of cathodoluminescent material 19, the scattered charged particles end up at a position on the layer of cathodoluminescent material 19 adjacent to the position of the 0.sup.th order transmitted charged particles, as indicated in FIG. 5. When these scattered charged particles hit the layer of cathodoluminescent material 19, photons are created from an interaction volume 191. When using the photons created by these scattered charged particles to make an image of the sample, a dark field image is obtained.

(23) In order to distinguish between a 0.sup.th order transmitted charged particles and the scattered charged particles, a segmentation of the detection area of a detector 20 can be used, as for example shown in FIG. 6.

(24) In a first embodiment, the light sensitive area's of the detector 20 is arranged a shown in FIG. 6. Thus for each primary focussed charged particle beam there is a group 60, 60 of light sensitive area's, also denoted as pixels, comprising:

(25) a centre light sensitive area 61, 61 for detecting light generated by the 0.sup.th order transmitted charged particles, and

(26) a ring of light sensitive area's 62, 63, 64, 65, 62, 63, 64, 65 arranged around the centre light sensitive area 61, 61 for detecting light generated by the scattered charge particles.

(27) In a second embodiment, the detector 20 comprises a large number of pixels arranged in rows and columns, which allow to detect any pattern of generated light to be detected. From those pattern, the light generated from the individual primary focused charged particles 3, 3, 3 can be distinguished, because this light is arranged in a group of light spots. Each of such a group of light spots comprises a central part which originated from the interaction volume 190 of the 0.sup.th order transmitted charged particles, see for example FIG. 5. This central part may be surrounded by light spots which originated from the interaction volume 191 of the scattered charged particles. The signals from these various light spots within the various groups can be separated using appropriated subroutines for reading out and analysing the light signals from the pixels of the detector 20.

(28) Alternatively, specific pixels or groups of pixels may be allocated or assigned for example according to the pattern as shown in FIG. 6. The central pixel or group of pixels 61, 61 is assigned for detecting light generated by the 0.sup.th order transmitted charged particles, and the surrounding pixels or group of pixels 62, 63, 64, 65, 62, 63, 64, 65 are assigned for detecting light generated by the scattered charge particles.

(29) In an exemplary embodiment the layer of cathodoluminescent material 19 comprises Yttrium aluminum garnet (YAG, Y3Al5O12), which is a synthetic crystalline material of the garnet group. Although a YAG crystal layer provides a very homogeneous and well defined layer of cathodoluminescent material, a disadvantageous of a layer of YAG is that it also absorbs part of the generated light. In order to limit the amount of absorption of the generated light, the layer of cathodoluminescent material 19 is preferably a thin layer, preferably smaller or equal to a maximum depth of the interaction volume, which is arranged on top of and/or supported by a light transmitting support plate 200, as for example shown in FIG. 7A.

(30) In an alternative exemplary embodiment, the layer of cathodoluminescent material 19 is thinner than the maximum depth of the interaction volume, which is also referred to as the scattering range, as shown in FIG. 7B. The use of such a thin layer of cathodoluminescent material 19 is, that signals from higher energy charged particle beams at least substantially do not interfere.

(31) In addition or alternatively the layer of cathodoluminescent material 19 is covered with a charge conducting layer 300, as for example shown in FIG. 8. This charge conducting layer 300 is preferably arranged at a side of said layer of cathodoluminescent material 19 facing the electro-magnetic lens system 13, 14, and preferably is connected to ground potential. The charge conducting layer 300 comprises for example a layer of ITO and/or graphene. Alternatively or in addition, the charge conducting layer 300 comprises a thin metal layer, for example having a thickness of 10 to 20 nm. Such a thin metal layer provides the desired charge conducting properties and in addition provides a mirror effect for the photons created in the cathodoluminescent material 19, which mirror effect can increase the part of the created photons which are directed toward the optical arrangement 40 and the detector 20.

(32) It is noted that the optical arrangement 40 comprises a lens system 41, 42 which is arranged for imaging said created photons 30, 31 onto the multi-pixel photon detector 20. Preferably the optical arrangement 40 provides an optical magnification between 5 and 500.

(33) In order to obtain an image from the surface of the sample 15, the apparatus further comprising a system 18 for scanning the focused primary charge particle beams over said sample, and a control-and-signal-processing system 21 for controlling the scanning system 18 and the detector 20, and for analyzing the data from the detector 20 and creating one image per primary charged particle beam.

(34) Alternatively or in addition, the apparatus further comprises a first actuating system 180 for moving the sample holder 150 at a constant speed in a first direction. Together with the scanning system 18 as a second actuating system, for scanning the focused primary charged particle beams over said sample in a second direction at least substantially perpendicular to the first direction, the focused primary charged particle beams can be scanned over an area of the surface of the sample 15. Again the control-and-signal-processing system 21 is for example arranged to create one image per primary charged particle beam.

(35) The control-and-signal-processing unit 21 is preferably arranged and/or provided with appropriate subroutines for combining said individual images per primary charged particle beam into one combined image of at least a part of the sample 15.

(36) FIG. 9 shows a further example of a detection arrangement according to the invention. This detection arrangement comprises substantially the same detection arrangement as shown in FIG. 4 and as discussed above. However, in this example, the optical assembly 40, 40 comprises a filtering device 120 which blocks light generated by the 0.sup.th order transmitted charged particles and/or light generated by the scattered charged particles, in particular in the layer of cathodoluminescent material 19. After the filtering device 120, the optical assembly 40, 40 comprises a further lens system 41, 42 which is arranged for imaging photons transmitted through the filtering device 120 onto the multi-pixel photon detector 20.

(37) When the photons created by the 0.sup.th order transmitted charged particles are blocked by the filtering device 120 and the photons created by the scattered charged particles are allowed to pass the filtering device, a dark field image is obtained.

(38) When the photons created by the scattered charged particles are blocked by the filtering device 120 and the photons created by the 0.sup.th order transmitted charged particles are allowed to pass the filtering device, a bright field image is obtained.

(39) For example, the filtering device 120 may comprise a set of special filters with apertures which are configured to either allow the light created by the 0.sup.th order transmitted charged particles to pass, or allow the light created by the scattered charged particles to pass. Alternatively, the filtering device 120 may comprise a programmable filter, for example using an array of LCD cells which are individually controllable to be transparent or opaque.

(40) It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention.